[Amps] Advice
Gary Schafer
garyschafer at comcast.net
Sun Dec 6 14:39:38 PST 2009
In looking more into other spec sheets it seems the cooling requirements are
all over the place. I have a chart in one book that shows a 4CX350 and the
cooling air requirements is almost linear with the back pressure going up
about 5 times with a doubling of air flow.
I see also that the 3CX800 does not seem to be linear.
What I was doing in the first post was comparing an 8877 at 1500 watts plate
dissipation to a pair of 3CX800s at 1600 watts dissipation. both would
require just about the same amount of air flow.
But it looks like if a pair of 3CX800s are run at a total of (1/2 for each
tube) dissipation then there would be less air flow required for the pair
than a single 8877, but then maybe not as we don't have the air flow
requirement for an 8877 at 800 watts plate dissipation. It may be the same!
Here is another one: A YC156 run at 2000 watts plate dissipation requires
only 35 cu ft of air at .1 inches back pressure.
Compare that to an 8877 at 1500 watts which requires 38 cu ft at.6 inches
back pressure.
Now use 2x 8877s for 3000 watts dissipation and you need 76 cu ft at .6
inches.
The YC156 at 3000 w plate dissipation requires 75 cu ft at .3 back pressure.
In this case the single tube would be easier to cool.
73
Gary K4FMX
> -----Original Message-----
> From: Joe Subich, W4TV [mailto:lists at subich.com]
> Sent: Sunday, December 06, 2009 12:41 AM
> To: 'Paul Decker'; garyschafer at comcast.net
> Cc: amps at contesting.com
> Subject: RE: [Amps] Advice
>
>
>
> The current Eimac data sheet gives the cooling requirements
> at sea level as:
>
> Anode CFM Pressure
> Diss In. Water
> ---------------------------------
> 400 6 0.09
> 600 11 0.20
> 800 19 0.50
>
>
> However, the original (1983) Eimac data sheet for the 3CX800
> gave the following:
>
> Anode CFM Pressure
> Diss In. Water
> ---------------------------------
> 800 19 0.35
>
>
> > The cooling requirements are the same per watt of dissipation
> > for the 8877 as they are for the 3CX800.
>
> Not according to the Eimac data sheet (1971) for the 8877
>
> Anode CFM Pressure
> Diss In. Water
> ---------------------------------
> 500 7.7 0.10
> 1000 20.3 0.23
>
>
> Each of the points has a different cfm/W value:
> 6/400 <> 19/800 <> 7.7/500 <> 20.3/1000
>
> 73,
>
> ... Joe, W4TV
>
>
> > -----Original Message-----
> > From: amps-bounces at contesting.com
> > [mailto:amps-bounces at contesting.com] On Behalf Of Paul Decker
> > Sent: Sunday, December 06, 2009 12:11 AM
> > To: garyschafer at comcast.net
> > Cc: amps at contesting.com
> > Subject: Re: [Amps] Advice
> >
> >
> >
> >
> > Hi Gary,
> >
> >
> >
> > I don't know if it's a linear relation. Perhaps a typo, but
> > on page 13.19 of the 1995 handbook, it says 12 CFM at 0.09"
> > back pressure for two 3cx800's operating at 400 Watts
> > dissipation each.
> >
> >
> >
> > Paul (KG7HF)
> >
> >
> > ----- Original Message -----
> > From: "Gary Schafer" <garyschafer at comcast.net>
> > To: "Paul Decker" <kg7hf at comcast.net>
> > Cc: amps at contesting.com
> > Sent: Saturday, December 5, 2009 11:57:34 PM GMT -05:00
> > US/Canada Eastern
> > Subject: RE: [Amps] Advice
> >
> >
> >
> >
> > Hi Paul,
> >
> > I didn't mean that I didn't understand the 2 nd paragraph but
> > that I didn't read it. I just overlooked it when I read your
> > first post with it.
> >
> >
> >
> > I do think that you have the cooling requirements wrong for
> > the 3CX800 though. I got the info from the 2000 ARRL handbook
> > section on amplifiers. There is a table there that list the
> > cooling requirements.
> >
> > The cooling requirements are the same per watt of dissipation
> > for the 8877 as they are for the 3CX800.
> >
> >
> >
> > By the way I see that I made a type in one place where I show
> > .03333 cu ft/watt. It should be .02333 cu ft/watt.
> >
> >
> >
> > At 400 watts dissipation each they should require around
> > (.02333 x 400) 9.3 cu ft of air each or 18.6 for a pair for
> > 800 watts dissipation.
> >
> >
> >
> > A single 3CX800 operated at 800 watts dissipation would
> > require 19 cu ft of air per the chart at .5 back pressure.
> >
> >
> >
> > 73
> >
> > Gary K4FMX
> >
> >
> >
> >
> >
> >
> >
> > From: Paul Decker [mailto:kg7hf at comcast.net]
> > Sent: Saturday, December 05, 2009 6:26 PM
> > To: garyschafer at comcast.net
> > Cc: amps at contesting.com
> > Subject: Re: [Amps] Advice
> >
> >
> >
> >
> > Hi Gary,
> >
> > The point of the second paragraph was that with a pair tubes
> > operating under their specificed dissipation rating require
> > less cooling then a single tube operating at or above its
> > dissipation range.
> >
> >
> >
> > Two 3cx800's operating at legal limit only require 12 CFM at
> > about 0.09 - 0.1" of back pressure, this is because the tubes
> > are only operating at 400W dissipation each.
> >
> >
> >
> >
> >
> > Paul Decker (KG7HF)
> >
> >
> > ----- Original Message -----
> > From: "Gary Schafer" <garyschafer at comcast.net>
> > To: garyschafer at comcast.net, "Paul Decker"
> > <kg7hf at comcast.net>, amps at contesting.com
> > Sent: Saturday, December 5, 2009 5:11:10 PM GMT -05:00
> > US/Canada Eastern
> > Subject: RE: [Amps] Advice
> >
> > Paul, sorry I missed your second paragraph.
> > Air flow needed depends on how much power is being
> > dissipated. If you divide
> > the airflow by the dissipation rating of the 8877 as an
> > example 35/1500 =
> > .02333 cu ft/watt. multiply that by 1600 (for a pair of
> > 3CX800s) and you get
> > 37.3 cu ft of air flow. The cooling efficiency of both are
> > about the same
> > .02333 cu ft/watt.
> >
> > So if you ran the pair of 3CX800s at the same power level as
> > the 3CX1500 it
> > would require just about the same amount of air flow. .03333
> > x 1500 on the
> > pair of 3CX800s = 35 cu ft.
> >
> > I don't know what the back pressure would look like on the
> > pair of 3CX800s
> > at reduced air flow but it would probably be pretty close to
> > the 3CX1500.
> >
> > 73
> > Gary K4FMX
> >
> > > -----Original Message-----
> > > From: amps-bounces at contesting.com
> > [mailto:amps-bounces at contesting.com]
> > > On Behalf Of Gary Schafer
> > > Sent: Saturday, December 05, 2009 4:40 PM
> > > To: 'Paul Decker'; amps at contesting.com
> > > Subject: Re: [Amps] Advice
> > >
> > > The chart that I am looking at says: 8877 requires 35 cu ft at .41
> > > back
> > > pressure.
> > >
> > > A 3CX800A7 requires 19 cu ft at .5 back pressure. Two of those would
> > > require
> > > 38 cu ft at .5 back pressure.
> > >
> > > 73
> > > Gary K4FMX
> > >
> > >
> > > > -----Original Message-----
> > > > From: amps-bounces at contesting.com
> > [mailto:amps-bounces at contesting.com]
> > > > On Behalf Of Paul Decker
> > > > Sent: Saturday, December 05, 2009 4:26 PM
> > > > To: amps at contesting.com
> > > > Subject: Re: [Amps] Advice
> > > >
> > > >
> > > >
> > > > When I was writing the original reply I was thinking more
> > along the
> > > > lines of different tubes for example a single 8877 requires more
> > > airflow
> > > > and has more back pressure than two 3cx800's.
> > > >
> > > >
> > > >
> > > > Agreed, two tubes of the same type would require more
> > airflow than a
> > > > single tube. However, I think we could assume that two
> > tubes of the
> > > > same type would produce 2x the power of the single tube
> > amp. If the
> > > > twin tube amp were run at the same output as the single holer, it
> > > would
> > > > also follow to reason the airflow requirements could for the two
> > > > holer
> > > > could be reduced because each tube is only being driven
> > to a reduced
> > > > output.
> > > >
> > > >
> > > >
> > > > Paul
> > > >
> > > >
> > > >
> > > >
> > > > ----- Original Message -----
> > > > From: "Gary Schafer" < garyschafer at comcast.net >
> > > > To: < dezrat1242 at yahoo.com >; < amps at contesting.com >
> > > > Sent: Saturday, December 05, 2009 11:04 AM
> > > > Subject: Re: [Amps] Advice
> > > >
> > > >
> > > > There is no free lunch. While it may seem like you could
> > get by with
> > > > less
> > > > air flow it doesn't follow.
> > > > A pair of tubes will require twice the air flow at the same back
> > > > pressure as
> > > > a single tube. Tubes being the same of course.
> > > >
> > > > 73
> > > > Gary K4FMX
> > > >
> > > >
> > > >
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